Raheem Sterling, centre, made 16 starts early in the season but played for Liverpool's Under-21s on Monday. Photograph: Andrew Powell/Liverpool FC via Getty

Brendan Rodgers has said he fully expected Raheem Sterling to suffer a dip in form as the effects of a debut Premier League season impact on the Liverpool winger.

The 18-year-old made a rare appearance for Liverpool's Under-21s on Monday night in a game against Manchester United that has become subject to a Football Association investigation. A United supporter confronted the Liverpool striker Adam Morgan on the pitch at St Helens' Langtree Park stadium and two flares were set off in the stands. There were also reports of distasteful chants from visiting supporters, although Merseyside Police received no complaints. The FA will write to Liverpool and United for their observations.

Liverpool lost the game 1-0, with Sterling playing as Rodgers looks to continue his development out of the limelight. Sterling enjoyed an eye-catching start to his senior Liverpool career, making 16 consecutive Premier League starts from August and his full England debut against Sweden in November, though his contribution has waned in recent months. His manager insists that is no surprise given the gruelling nature of the Premier League.

"This is a kid we're talking about so we have to take a wee step back," said the Liverpool manager. "What he's done since he stepped onto the pitch against Bayer Leverkusen in a friendly to Manchester City [for his league debut] as a 17-year-old, I dare you to look around Europe and tell me a 17-year-old who has made such an impact in one of the big leagues. There aren't many.

"It is so mentally and physically draining this league. It hurts experienced professionals and this boy has just stepped up from the Academy and played first-team football and earned a contract off that. We have to nurture that and at times he's got to have that mentality to keep learning."

Only Steven Gerrard and Luis Suárez have made more first-team appearances for Liverpool this season than Sterling, who has started five of the last 14 matches and faced greater competition from January signings Daniel Sturridge and Philippe Coutinho plus an improved Stewart Downing.

Rodgers added: "I was talking to him the other day about this, and if he didn't kick another ball this season he's had a good one. It's a very physically demanding league and what you're seeing now is more tiredness of the legs when before he could go up to people and take them on. That zip has gone and that's down to fatigue. Was that to be expected? Yes it was.

"He will have experienced that before but not in the limelight like it is now. He's made such an impact that everyone is looking at him now. But the job is to nurture him. He's started his career at first-team level way before many get the opportunity. He's still got lots of improvement to make in his game. Over the years we will look at his plan and development and act accordingly."